I understand (correct me if Im wrong) that Mr. Robot will implement vertex shaders - I was wondering, what kind of hardware will a VS run on these days, with good performance? Is it worth using vertex shaders (even in a 2d game)?

If you run a fixed function DX program (no vertex shaders) on very old hardware then operations that cannot be hardware accelerated are executed on the CPU i.e. software transform. If you write a DX program and do use vertex shaders then on old hardware this will also execute on the CPU. So in some situations, whether you use vertex shaders or not, you are running code on the CPU per vertex. So deciding to use a vertex shader isn't necessarily a bad thing as old/cheap hardware will execute some or all of your code on the CPU regardless.

Now the reality is that the fixed function pipeline has been around a long time, so if anything is going to get hardware accelerated, driver optimised and be fast then that is it (especially if we are talking Intel Extreme - yuck!). So for best performance and widest support, the fixed function pipeline is your friend. Vertex shaders can be very handy for custom work and if it isn't performance critical then go for it, on low end hardware it will execute on the CPU but so what. It kind of depends on what minimum spec you are aiming at and what kind of performance you need. We always write little test apps and have a range of very poor PC's on hand to test things out on.